The invention relates to a generator for an engine having a generator housing that houses a stator and rotor, and a cover plate that includes a bearing for the rotor shaft and is flanged to a gearbox casing.
Generators flanged to the gearbox casing via a cover plate have been known for a long time but they cause problems because the functioning of the generator is adversely affected and its service life significantly reduced by the considerable quantity of heat produced by its rotor and stator. Engineers have tried to solve this problem by improving heat dissipation from the generator housing via its cover plate to the gearbox casing, in that the cover plate was made of a material with high thermal conductivity, like the aluminum alloy used here, so that the heat can be dissipated from the generator housing (that is also made of an aluminum alloy) via the cover plate into the gearbox casing.
The rotor shaft is typically supported by a steel ball bearing that is integrated into the cover plate. If the cover plate involves two materials with different heat conductivities, there is the disadvantage that the steel bearing of the rotor shaft cannot be incorporated firmly enough into the aluminum cover plate, since the two materials differ in thermal expansion. As the aluminum cover plate shows comparatively little stiffness and does not meet the dynamic requirements of high rotor speeds and a durably stable seat of the bearing in the cover plate, the bearing of the rotor shaft does not have a long service life. An insert made of a high-strength and rigid material such as a titanium alloy must be used in the cover plate. An increased clearance can result between the shell of the bearing and the casing results in jams and vibrations, which eventually damages the bearing, the stator and the rotor.
The proposal to make the entire cover plate of a high-strength material such as steel or titanium has solved the problem described above and represents today's state of the art. However, this results in comparatively poor heat conductivity and excessive operating temperatures of the generator stator, leading to increased failure rates of these generators. The proposal to create as large a heat transfer surface as possible by making the cover plate large-volume is doomed to failure as this increases the volume and weight of the cover plate, the latter effect being particularly undesirable in aircraft engines. Cooling fins on the generator housing would be an excellent alternative solution for heat dissipation in normal operating conditions but fail here because in the event of a fire a greater heat input with unchanged poor thermal conductivity towards the gearbox will result in early generator failure, which prevents it from meeting the five-minute minimum functionality requirement in the event of a fire to supply power to the electronic control module and overspeed protection unit for safe engine shutdown.
It is an object of this invention to design a generator flanged to the gearbox casing of an engine in such a way that it ensures an exactly centered bearing of the rotor shaft and exact alignment of the rotor with the stator while as great a heat quantity as possible is dissipated from the stator housing for a long service life of the generator.